This invention related to a process for anodizing aluminum and its alloys with a hard finish. More particularly this invention relates to a process for hard anodizing of aluminum and its alloys wherein the anodizing electrolyte is enhanced by the addition of lignin or a lignosulfonate salt.
It is well known in the art to provide for the anodic oxidation of aluminum and its alloys with an acid electrolyte such as sulfuric acid, oxalic acid, chromic acid, sulfamic acid and phosphoric acid to produce a hard corrosion resistant layer. More recently it has been proposed to utilize an addition along with the anodizing electrolyte. For example U.S. Pat. No. 2,855,350 utilizes 2-aminoethyl sulfuric acid as an additive to allegedly produce a thick dense hard oxide coating. Similarly U.S. Pat. No. 2,855,351 adds taurine (2-aminoethane sulfuric acid) and amino substituted taurines to also allegedly produce thicker, denser and harder oxide coatings than are produced by the conventional acid electrolyte. The next consecutively numbered patent U.S. Pat. No. 2,855,352 issued to the same inventor and assignee claims that sulfamic acid (NH.sub.2 SO.sub.2 H) and its substituents also produce the same effects as the previously mentioned patents.
About the same time a different invention having the same assignee alleged in U.S. Pat. No. 2,897,125 that a surface active agent added to the electrolyte gave improved results with an aqueous extract of lignite, brown coal or peat. The extract is stated on column 3, line 25 to have 4.5 to 5% nitrogen content. Similarly in U.S. Pat. No. 2,905,600 the same inventor discovered improved anodization with a peat extract containing 4.5 to 5% nitrogen in two steps anodizing process.
In a continuation-in-part U.S. Pat. No. 2,977,294 the same inventor utilized a two bath anodization process wherein the first bath contains a peat extract containing 4.5 to 5% nitrogen residue.
It is claimed in U.S. Pat. No. 3,434,943 that superior anodizing results are obtained by adding to the electrolyte bath an extract obtained by treating the quebracho tree with water. The patentee allegeded that this extract contained a derivative of orthodihydroxy benzine but did not wish to be held to any formula. According to The Condensed Chemical Dictionary, Sixth Edition, the chief constituents of the quebracho tree are aspidospermine C.sub.22 H.sub.30 O.sub.2 N.sub.2 which is an alkaloid, tannin C.sub.14 H.sub.10 O.sub.4 and yohimbrine C.sub.21 H.sub.26 O.sub.3 N.sub.2. Aspidospermine and yohimbine are both alkaloids.
A common functional or active group in all the above mentioned additives is nitrogen. Many of the compounds contain an amino group. Peat contains 2 to 3% ammonia and about 22% humus acids which have soluble nitrogen compounds.
While these compounds have been somewhat effective as additives, there are others which are superior.